1. Field of the Invention
The present invention relates to an optical element in which a refractive index pattern that is formed by a plurality of media having refractive indices different from each other is periodically formed.
2. Description of the Related Art
Recently, high functionality of a solid-state image sensor is desired, and therefore an element using a photonic crystal that obtains a sufficient function even when the size of a pixel is around the same as a wavelength of a sensing electromagnetic wave is proposed. The photonic crystal is a structure that has a periodically-varying refractive index. Commonly, the period of the structure of the photonic crystal is around the same as the wavelength of the mutually-interacting electromagnetic wave. In addition, the structure of the photonic crystal is commonly designed so as to be optimized for each wavelength of the electromagnetic wave and have a selective effect for a specific wavelength.
Japanese Patent Laid-Open No. 2005-142429 discloses a configuration in which red, green, and blue photonic crystals are used for red, green, and blue wavelengths respectively and an electromagnetic wave (light) vertically enters a microlens for each of the red, green, and blue wavelengths. Japanese Patent Laid-Open No. 2005-79674 discloses a technology in which a photonic crystal is used to emit red, green, and blue lights at different angles to sense the light using solid-state image sensors having three different substrates for the red, green, and blue lights. This technology relates to a solid-state image pickup apparatus of a three-substrate type, and controls propagation directions of the red, green, and blue lights. Japanese Patent Laid-Open No. 2006-221976 discloses a diffraction grating that functions for all of the red, green, and blue wavelengths and that has a structure smaller than a wavelength. In the technology disclosed in Japanese Patent Laid-Open No. 2006-221976, diffraction efficiencies of the red and blue lights are controlled by using the same diffraction grating. Japanese Patent Laid-Open No. H09-105927 discloses a liquid crystal device that suppresses the deterioration of light use efficiency that is generated by a color filter used in performing the color separation of white light that is visible light into red, green, and blue lights using a diffraction grating.
However, in the configuration disclosed in Japanese Patent Laid-Open No. 2005-142429, the light use efficiency is deteriorated by cutting the light having other wavelengths using the color filter in sensing the lights having the red, green, and blue wavelengths. In addition, a phenomenon called a false color in which an image is taken by a color different from an original object color is generated.
Japanese Patent Laid-Open No. 2005-79674 cannot sense the red, green, and blue lights at a fine period since the propagation directions of the red, green, and blue lights are controlled to be sensed by the three-substrate solid-state image sensor.
In Japanese Patent Laid-Open No. 2006-221976, an area ratio of two kinds of diffraction gratings that correspond to the red and blue lights is adjusted to control the diffraction efficiencies of the red and blue lights, but the number of variables that can be independently adjusted is small when using only the area ratio of the diffraction gratings and therefore it is difficult to maximize the diffraction efficiency. Furthermore, it does not disclose a method of independently controlling the diffraction efficiency of the green light. In Japanese Patent Laid-Open No. 2006-221976, the shape of the structure is not adjusted although the grating period is adjusted, and therefore the light intensity distribution for each wavelength cannot be controlled with higher precision.
In Japanese Patent Laid-Open No. H09-105927, the propagation direction of each of the red, green, and blue lights is controlled, but a relative value of the diffraction efficiencies of a plurality of diffraction orders is not controlled. Therefore, each light intensity distribution cannot be precisely controlled.